1. Field of the Invention
The present invention relates to a numerical controller for controlling a multi-axis machine tool (multi-axis processing machine) for machining a workpiece (item to be machined) attached to a table using a mechanism for rotating a table or tool head using at least three linear axes and three rotating axes, and in particular to a numerical controller including a workpiece setting error compensation unit for compensating a setting error that arises when a workpiece is set.
2. Description of the Related Art
Five-axis machine tools having two rotating axes in addition to three linear axes (X, Y, and Z axes) are coming into widespread use. Numerical controllers including a workpiece setting error compensation unit for compensating a workpiece setting error that may arise when a workpiece is set in such a five-axis machine tool are disclosed, for example, in the following three patent references.
Japanese Patent Application Laid-Open No. 2009-15464 discloses a technique for compensating a workpiece setting error that arises when a workpiece is set in a five-axis machine tool by converting an interpolated position on a machine coordinate system into a position on an intermediate coordinate system and converting the position on the intermediate coordinate system into a position on the machine coordinate system. This technique is, however, inapplicable to a numerical controller because this patent reference does not describe which solution should be selected out of a plurality of solutions for rotating axis positions.
U.S. Pat. No. 8,041,447 discloses a technique for calculating tool position and direction on a command coordinate system in a five-axis machine tool and selecting a solution giving a position closer to the commanded position if there are a plurality of solutions when rotating axis positions are calculated after a workpiece setting error is compensated.
Japanese Patent Application Laid-Open No. 2011-133968 discusses a problem with the above-mentioned U.S. Pat. No. 8,041,447 in that rotating axes may largely move near singular points. Large motions of rotating axes near singular points would result in disadvantages such as an increased machining time and a degraded machined surface such as an uneven and/or streaked machined surface. Japanese Patent Application Laid-Open No. 2011-133968 describes, therefore, a technique for causing rotating axes to pass through singular points when the rotating axes are commanded to move over the singular points to the opposite side. This technique, however, causes a machining error because of a changed tool path. These techniques are all directed to five-axis machine tools as describe above.
Recently, tool heads equipped with three rotating axes as shown in FIGS. 1 and 2 are coming into use for head rotation type multi-axis machine tools. Mixed type multi-axis machine tools having two rotating axes for rotating the table and one rotating axis for rotating the head (see FIG. 3) and mixed type multi-axis machine tools having one rotating axis for rotating the table and two rotating axes for rotating the head (see FIG. 4) are also coming into use. Furthermore, a table rotation type multi-axis machine tool having three rotating axes for rotating the table is contemplated as a machine configuration (see FIG. 5). The conventional techniques relate to a five-axis machine tool having three linear axes (X, Y, and Z axes) and two rotating axes as described above and there has been no technique for a numerical controller to compensate a workpiece setting error when a workpiece is set in a multi-axis machine tool having three rotating axes.
Although the aforementioned U.S. Pat. No. 8,041,447 refers to selecting one of a plurality of solutions that exist, but either one of the two solution sets is merely selected. As described later, in a multi-axis machine tool having three rotating axes, there are innumerable solutions when positions of the three rotating axes are calculated. The conventional techniques relating to the above five-axis machine tools are, therefore, difficult to apply to a multi-axis machine tool having three rotating axes.
Furthermore, five-axis machine tools have problems including large motions of rotating axes near singular points and machining errors as described above.